Preheat an oven, then line a rimmed baking sheet with parchment paper. Pat the salmon filets dry on all sides with paper towels, then season with Kosher salt.
In a small bowl, whisk together Kewpie mayo and Sriracha until well mixed. Reserve half of the sauce for serving. Brush the remaining spicy mayo on top of each salmon filet.
Place mixed white and black sesame seeds in a shallow bowl, then stir to mix. Firmly press each piece of salmon, mayo-side down, into the seeds. Transfer, seed-side up, to the prepared baking sheet.
Transfer to the oven and cook until the salmon is fully cooked.
Serve with the miso dipping sauce on the side and lots of sliced green onions, cilantro, and lime wedges. Additional topping ideas: pickled ginger, sliced cucumber, and avocado!